In LTE up to release 12, only orthogonal multiple access (OMA) is used where user equipments (UEs) are multiplexed in either time, frequency or spatial domain or a combination of time, frequency and spatial domains. Another form of UE data multiplexing under study in LTE release 13 is multi-user superposition transmission (MUST), in which two or more UEs with different path losses to a serving base station are superposed on the same time-frequency and/or spatial resources, such as Orthogonal Frequency Division Multiplexing (OFDM) resource elements. This is realized by assigning different transmit powers to the different UEs. The total power is split among the UEs served in the same time-frequency resources, where the transmit power level allocated to a given UE (or ‘power share values’) is generally determined by the channel condition (i.e., path loss) experienced by the UEs. For instance, UEs having higher path loss (i.e. UEs far away from the eNodeB, or eNB) can be allocated higher transmit powers while UEs having lower path loss (i.e., UEs near to the eNB) can be allocated lower transmit powers. The total combined transmit power is, however, kept the same.
An example is shown in FIG. 1, where a near UE (UE1) 10 and a far UE (UE2) 20 are present in a cell 40 served by a radio access network node, such as eNB 30. The two UEs 10 and 20 can be superposed at the same time-frequency resource as follows:x=√{square root over (P1)}s1+√{square root over (P2)}s2  eq. (1)where x is the superposed signal transmitted from the eNB 30, Pi is the allocated transmit power to UEi (i=1, 2) and
                    ∑        i            ⁢              P        i              =    P    ,where P is the total transmit power over the resource element. The received signal at UE i is thenyiHi·(√{square root over (P1)}s1+√{square root over (P2)}s2)+vi  eq. (2)oryi=Hi·√{square root over (P)}(√{square root over (α1)}s1+√{square root over (α2)}s2)+vi  eq. (3)where Hi (i=1, 2) is the channel response to UEi,
            α      1        =                                        P            1                    P                ⁢                                  ⁢        and        ⁢                                  ⁢                  α          2                    =                        P          2                P              ,vi (i=1, 2) is the receiver noise at UEi.
FIG. 2 shows the received signal power at each of the UEs. UE1 10 is closer to the eNB 30 (i.e. a cell center UE) and thus has a smaller propagation path loss, while UE2 20 is far away from the eNB 30 (i.e. a cell edge UE) and thus has a larger propagation path loss. To reach UE2 20, a higher transmit power is needed than for UE1 10, i.e. P2>P1. When P1 is much smaller than P2, UE2 20 is still able to decode its data successfully at the presence of UE1's 10 signal. Since UE1 10 is close to the eNB 30, it would see a strong signal 12 intended for UE2 20. If UE1 10 can estimate the signal 12 H1·√{square root over (P2)}s2, then it can cancel this estimate from the received signal y1. After the cancellation, UE1 10 would be able to decode its own signal.
General MUST transmitter and receiver diagrams are shown in FIGS. 3 and 4, respectively. For example, FIG. 3 shows a simplified block diagram of a MUST transmitter configured to superpose transmitted symbols for two UEs. As shown in the figure, the information bits 302 corresponding to the near UE1 10 (i.e. the cell-center UE) and those 312 corresponding to the far UE2 20 (i.e. the cell-edge UE) are first separately channel encoded 304, 314. The two sets of channel encoded bits are then jointly modulated 306, 316 and precoded 308, 318 with the appropriate transmit power level settings to produce the MUST signal 320. Generally, a higher transmit power level is allocated to the far UE2 20 and a lower transmit power level is allocated to the near UE1 10. The total transmit power is kept unchanged.
FIG. 4 shows a simplified block diagram of MUST receiver processing for a case with two superposed UEs. Since the two UEs are allocated different power levels, the near UE1 10 can attempt to cancel the interference emanating from the data transmission intended to the far UE2 20.
The MUST signal 320 is received at the near UE1 10 and the far UE 20, shown by flow blocks 402, 412. Typically, the far UE2 20 uses a normal receiver and need not even be aware that there is a superposed transmission to a near UE1 10. The interference cancellation for the near UE1 10 can be done in two ways. A first option is that the codeword corresponding to the far UE2 20 is decoded at the near UE1 10 and then reconstructed 404 and cancelled or removed from the received signal. This type of cancellation is referred to as codeword level interference cancellation (CWIC). A second option is that the near UE1 10 makes a symbol-wise hard demodulation decision of the symbols corresponding to the far UE2 20 and then cancels the interference. This type of interference cancellation is referred to as symbol level interference cancellation (SLIC).
Following the steps of interference cancellation, the near UE1 10 then decodes 406 its own codeword(s), to generate a decoded data stream 408. For certain flavors of MUST schemes, a third option is also possible where the near UE1 10 collects its own coded bits (i.e. discards the far UE2 20 coded bits) and then proceeds towards decoding 406 its own codeword(s).
Given that the far UE2 20 is allocated a higher transmit power level than the near UE1 10, the far UE2 20 demodulates and decodes 416 its own codeword without cancelling the interference emanating from the data transmission intended for the near UE1, to generate a decoded data stream 418.
When the base station (BTS), such as eNB 30, has multiple transmit antennas, each of the signals can be precoded before transmission. In this case, the transmitted signal from a base station becomesx=√{square root over (P1)}W1s1+√{square root over (P2)}W2s2  eq. (4)where x=[x1, x2, . . . , xNTX]T and xn(n=1, . . . , NTX) is the transmitted signal on the nth antenna, NTX is the number of Transmit antennas; Wi(i=1, 2) is a NTX×1 precoding vector applied to the signal si. If the UEs also have multiple receive antennas, the received signal at UEi becomes:yi=Hi·x+vi=Hi·(√{square root over (P1)}W1s1+√{square root over (P2)}W2s2)+vi  eq. (5)where yi=[yi(1), yi(2), . . . , yi(NiRX)]T, yi(k) is the received signal on antenna k of UEi, NiRX is the number of receive antennas of UEi; Hi, is a NiRX×NTX channel matrix, and vi is a NiRX×1 noise vector. Similar to the single antenna case, if UE1 can, by using the channel estimate Ĥ1 and information about √{square root over (P2)}W2, estimate the transmitted signal √{square root over (P2)}W2s2, then UE1 is able to decode its own signal after subtracting Ĥ1·√{square root over (P2)}W2s2 from the received signal y1=H1·(√{square root over (P1)}W1s1+√{square root over (P2)}W2s2)+v1.
Three variants of MUST schemes are being considered in the Release 13 study item on MUST. Brief descriptions of these schemes are given below.
Non-Orthogonal Multiple Access (NOMA)
In the NOMA scheme, the information bits corresponding to the far UE2 20 and the near UE1 10 are independently encoded and modulated. The symbol s1 is drawn from a near UE1 constellation, and the symbol s2 is drawn from a far UE2 constellation. Then the superposed symbol x in the NOMA scheme has a superposed constellation (super-constellation). An example of the superposed NOMA constellation for the case where both the near UE1 10 and far UE2 20 employ QPSK constellation is shown in FIG. 5. In this case, the superposed constellation is similar to a 16 QAM constellation.
Semi-Orthogonal Multiple Access (SOMA)
SOMA differs from the NOMA scheme in that SOMA uses Gray mapped superposed constellation. The coded modulation symbols of near UE1 10 and far UE2 20 are jointly Gray mapped and then added together as in eq. (1). An example of the superposed SOMA constellation for the case where both the near UE1 10 and far UE2 20 employ QPSK constellation is shown in FIG. 6, where α=α1.
Rate-Adaptive Constellation Expansion Multiple Access (REMA)
REMA is similar to SOMA with one restriction that the resulting superposed constellation should be a regular QAM constellation having equal horizontal and vertical spacing between constellation points (as is used in, e.g., LTE). In REMA, the bits with the higher bit-level capacities are allocated for the far UE2 20 and the bits with the lower bit-level capacities are allocated for the near UE1 10. In addition, the power sharing parameter should also be set appropriately so that the resulting superposed constellation is a regular QAM constellation. There are six different ways (shown in the table of FIG. 7) of realizing REMA that has LTE standard constellations as superposed constellations. In an example, FIG. 8 illustrates an example of 16-QAM superposed REMA constellation.
Wideband Scheduling
With wideband scheduling, an eNB may schedule the whole available frequency resources (i.e. the whole frequency band) to either a UE using OMA transmission or multiple (e.g. two) UEs, each on the whole frequency band, using MUST transmission in each subframe. The scheme selected depends on whether or not a suitable UE pair can be found for MUST scheduling based on some scheduling metric, e.g. proportional fairness (PF) metric. If there is a suitable pair of UEs found in a subframe, MUST transmission may be scheduled. Otherwise, OMA transmission may be scheduled. An example is shown in FIG. 9, where a suitable pair (UE1 and UE2) are found and MUST is scheduled for these two UEs in subframe (k+2). In the remaining subframes, OMA transmission is scheduled.
MUST transmission is scheduled if it gives higher average performance than OMA transmission, according to an adopted scheduling metric. A candidate user set for MUST transmission (also referred to as a UE candidate set) includes two or more UEs to be scheduled on the same resource. For each candidate user set for MUST transmission, the candidate transmission power set is searched and the transmission power allocation that gives the best performance is selected. Then, the candidate user set for MUST with the best performance is compared with the OMA user with the best performance, to determine what kind of scheme to use for scheduling. For wideband, a PF scheduling performance metric for MUST transmission is calculated as
            ∑              i        ∈        U              ⁢                            t          i                ⁡                  (          p          )                            T        i        a              ,where U is the candidate user set for MUST transmission, Ti is the historic average throughput for UEi, ti(p) is the estimated throughput of UEi when it is scheduled in the whole frequency band in the examined subframe based on a channel quality estimation of the UE being co-scheduled with other UEs in the candidate set, P denotes the allocated power set, and a is a PF coefficient used to balance data throughputs between cell edge and cell center UEs.
      ∑          i      ∈      U        ⁢                    t        i            ⁡              (        p        )                    T      i      a      will be referred to as a wideband PF scheduling metric for MUST.
In case of OMA scheduling, the wideband PF scheduling metric for UEi is calculated as
            t      i              T      i      a        ,where ti is the estimated throughput of UEi when it is the only UE scheduled in the whole frequency band in a subframe.Subband Scheduling
The frequency band can be divided into multiple non-overlapping subbands, each subband using different frequencies. With subband scheduling, an eNB may schedule either a UE using OMA transmission or multiple (e.g. two) UEs using MUST transmission in each subband in a subframe, depending on whether or not a suitable UE pair can be found for MUST scheduling in that subband and subframe. This may be based on some scheduling metric, such as PF. If there is a suitable pair of UEs found in that subband and subframe, MUST transmission may be scheduled, otherwise OMA transmission may be scheduled. As a subband PF metric, the performance for MUST transmission in a specific subband is calculated as
            ∑              i        ∈        U              ⁢                            t          i                ⁡                  (                      p            ,            f                    )                            T        i        a              ,where U is the candidate user set for MUST transmission, Ti is the historic average throughput for UEi, ti(p,f) is the estimated throughput if it is scheduled in subband f in the examined subframe, P denotes the allocated power sets, and a is the PF coefficient.
In the case of OMA scheduling in a subband, the subband PF scheduling metric for UEi is calculated as
                    t        i            ⁡              (        f        )                    T      i      a        ,where ti(f) is the estimated throughput of UEi when it is the only UE scheduled in subband f in the examined subframe.
In order to limit the receiver complexity, it is not desirable to have a UE perform interference cancellation in some subband(s) while not in other subband(s). For example, a UE cannot act as both a far UE and a near UE at the same time. Besides, allocating different transmission power to a UE for different subbands is not allowed. This implies the following restrictions.
As one restriction, once a UE is scheduled using MUST transmission and performs interference cancellation in one subband, it has to do so in all scheduled subband(s). The UE can be paired with multiple different UEs, but the power allocation must be the same over all scheduled subbands.
As another restriction, once a UE is scheduled using MUST transmission and does not perform interference cancellation in one subband (i.e. the UE is scheduled as a “far” UE in the subband), it has to be a “far” UE in all scheduled subband(s). While the UE can be paired with multiple different UEs, the power allocation must be the same over all scheduled subbands.
As a third restriction, once a UE is scheduled using OMA transmission in one subband, it has to be scheduled with OMA transmission in all scheduled subband(s), and with the same power allocation.
An example is shown in FIG. 10, with the restrictions described above. In subframe k and (k+3), both OMA transmission and MUST transmission are scheduled in different subbands. In subframe (k+1) and (k+2), MUST transmission is scheduled in all subbands. In subframe k+1, UE1 is paired with UE2 in subbands n+1 and n+2 and with UE3 in subband n. Similarly, in subframe k+2, UE3 is paired with UE1 in subbands n and n+2 but with UE2 in subband n+1. So generally, a UE may be paired with different UEs in different subbands.
The subband scheduling procedure is briefly described as such. In each subband and for each candidate user set for MUST transmission, the candidate transmission power set is searched and the transmission power allocation that gives the best performance is selected. Then, the candidate user set with the best performance is compared with the OMA user with the best performance to determine what kind of scheme to be scheduled in that subband.
For example, if there are three users UE1, UE2 and UE3} waiting to be scheduled, the candidate user sets for MUST transmission include {UE1, UE2}, {UE1, UE3}, and {UE2, UE3}. For a given candidate user set {UEi, UEj} (i,j=1, 2, 3), the transmission power sets are the possible power allocations between the two UEs, for example, {(0.9, 0.1), (0.8, 0.2), (0.7, 0.2), (0.6, 0.4), (0.4, 0.6), (0.3, 0.7), (0.2, 0.8), (0.1, 0.9)}.
When this is finished, a UE may be allocated on multiple subbands and an associated transmission power on each subband. Transmission power allocation is optimized for each subband, which however may be different from subband to subband for the same UE. In addition, the UE may be paired with another UE in one subband using MUST, while not paired with any UE in another subband using OMA. Furthermore, the UE may be treated as a near UE in one subband (in which case, interference cancellation is needed), while treated as a far UE in another subband (in which case, no interference cancellation is required) for MUST transmission. Therefore, whether there exists a paired UE or not (MUST or OMA), and whether the interference cancellation is needed or not, may be different in the scheduled subbands for a certain UE. That is, a conflict may exist. The conflict occurs when the different types of scheduling of a UE (i.e. any two of MUST as a near UE, MUST as a far UE and OMA) occur at the same time, e.g. in the same subframe. For example, as shown in FIG. 11, UE #3 is required to cancel the interference 1102 from UE #2 in subband #4, while not required 1104 in subband #5, #6, and #8. UE #3 is scheduled for OMA transmission 1106 in subband #5, and UE #1 is scheduled for OMA transmission in subband #7. Further steps are needed to meet the restrictions mentioned above, such as subband releasing.
Step 1: For a UE of interest, if the number of subbands requiring interference cancellation is the largest, those subbands are retained as the scheduled subbands requiring interference cancellation and other subbands for this UE are released 1108 (Step 2). For a UE of interest, if the number of subbands not requiring interference cancellation is the largest, those subbands are retained as the scheduled subbands not requiring interference cancellation and other subbands for this UE are released 1108. For a UE of interest, if the number of subbands not having the paired UEs is the largest, those subbands are retained as the scheduled subbands not having the paired UEs, i.e., OMA subbands, and other subbands for this UE are released 1108.
Step 3: Perform rescheduling 1110 and repeat steps 1-2 for the released subband(s), until no subband is to be released. Note that in each round of re-scheduling, the best UE for each of the released subbands has to be re-searched based on the scheduling metric, or the sorting of UEs needs to be performed based on the scheduling metric in the first round of scheduling for all the subbands, instead of just finding the best. This implies higher computation costs.
Step 4: Align the transmission power allocation for each UE in the scheduled subbands 1112. This requires one more search of the candidate transmission power set with the restriction that the same power should be allocated for different scheduled subbands for a UE.
Due to the need for subband release and the multiple iterations of re-scheduling, together with the conflict resolving, the subband scheduling with MUST may bring a much higher complexity than the wideband scheduling and the subband scheduling with OMA. With MUST, the complexity increase with wideband scheduling is moderate compared to a wideband scheduling with only OMA transmission. However, the performance improvement may be impacted. It has been shown that MUST with wideband scheduling could provide some gain over OMA with wideband scheduling at high load, but the gain over OMA with subband scheduling is much smaller, or even a loss. In other words, OMA with subband scheduling performs better than OMA with wideband scheduling. Therefore, to further improve system performance, MUST scheduling over subband is needed. On the other hand, MUST with subband scheduling has a much higher complexity compared to subband scheduling with only OMA transmission. This is especially a problem considering that MUST provides a gain at high load, where there are more users in the system to be scheduled. Furthermore, the subband scheduling that has been proposed may lead to sub-optimal performance, as the subband releasing is purely based on counting different types of subband scheduling. This does not take the actual performance into account, i.e., it is not guaranteed that the proposed scheduling will always provide a gain over the purely OMA subband scheduling. Besides, per user based subband release may lead to undesired behavior. For instance, as illustrated in FIG. 12, subband k to k+2 will be retained. Subband k+3 to k+6 will be released, as for both UE1 and UE2, the number of subbands having the paired UEs is the largest. However, if we treat UE1 and UE2 jointly, or specifically user pair based treatment, subband k+3 to k+6 will then be retained while subband k to k+2 will instead be released. The latter is actually what is desired, i.e., to retain the type of subband (MUST or OMA) with the largest number of scheduled subbands.